Income Taxes - Analysis of Deferred Tax Assets and Deferred Tax Liabilities Showing Tax Effects of Significant Temporary Differences (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets
Property and leasehold costs	$ 95,141	$ 233,351
Liabilities for dismantlements	28,475	78,361
Postretirement and other employee benefits	128,281	125,250
Alternative minimum tax	0	9,765
U. S. net operating loss	589,067	495,252
Investment in partnership	65,216	0
Other deferred tax assets	112,685	66,795
Total gross deferred tax assets	1,018,865	1,008,774
Less valuation allowance	(106,448)	(103,113)
Net deferred tax assets	912,417	905,661
Deferred tax liabilities
Deferred tax on undistributed foreign earnings	(5,000)	(5,000)
Accumulated depreciation, depletion and amortization	(665,255)	(938,614)
Deferred Tax Liabilities, Investments	0	(14,250)
Other deferred tax liabilities	(27,250)	(25,708)
Total gross deferred tax liabilities	(697,505)	(983,572)
Net deferred tax (liabilities) assets	$ 214,912
Net deferred tax (liabilities) assets		$ (77,911)